In an Orthogonal Frequency Division Multiplexing (OFDM) transmission method used in radio communication, signal transmission is performed with a plurality of subcarriers orthogonal to each other. Accordingly, with the OFDM transmission method, it is possible to achieve high frequency utilization efficiency and high-speed transmission. A transmission OFDM symbol sequence is divided into a plurality of transmission OFDM symbols, and these transmission OFDM symbols are transmitted in parallel with a large number of subcarriers. As a result, it is possible to reduce a transmission speed for a single subcarrier and reduce multipath delay interference.
In the OFDM transmission method, in order to reduce multipath delay interference, a guard interval (GI) is set between OFDM symbols. By setting a guard interval having a time length longer than a multipath delay time, it is possible for a receiver to exclude a portion affected by a delay wave from received OFDM symbols and demodulate these OFDM symbols.
FIG. 1 is a diagram describing a guard interval. A guard interval (Cyclic Prefix) is generated by copying the end portion of an OFDM symbol and pasting the end portion to the head portion of the OFDM symbol. Referring to FIG. 1, the end portion of an OFDM symbol #n+1 is copied and is then pasted to the head portion of the OFDM symbol #n+1 (a guard interval GI #n+1). At that time, while there is continuity between the signal of a target OFDM symbol (the OFDM symbol #n+1) and the signal of the guard interval GI #n+1, there is discontinuity between the signal of a preceding OFDM symbol (an OFDM symbol #n) and the signal of the guard interval GI #n+1. Such signal discontinuity generates a high-frequency component, the high-frequency component causes out-of-band power leakage, and the out-of-band power leakage becomes interference in an adjacent channel. Examples of processing for reducing a high-frequency component generated by signal discontinuity between OFDM symbols include Time Window processing (see, Japanese Laid-open Patent Publication No. 2008-11037).
FIGS. 2A to 2D are diagrams describing Time Window processing. In Time Window processing, discontinuity is eliminated by smoothly attenuating a target OFDM symbol and the next OFDM symbol between these OFDM symbols with a raised cosine waveform and adding the attenuated portions of these OFDM symbols so that the attenuated portions overlap each other. More specifically, as illustrated in FIG. 2A, when a Time Window width is Nwin, a portion (partial data a) having a width (Nwin/2) half the Time Window width Nwin is extracted from an OFDM symbol #n+1 and is then added to the head portion of a guard interval GI #n+1 and a portion (partial data b) having the width (Nwin/2) half the Time Window width Nwin is extracted from an OFDM symbol #n and is then added to the end portion of the OFDM symbol #n.
The end portion of the OFDM symbol #n including the partial data b having the Time Window width Nwin is multiplied by the coefficient of a Raised Cosine function. The head portion of the guard interval GI #n+1 including the partial data a having the Time Window width Nwin is multiplied by a window function (for example, a Raised Cosine function). By multiplying the end of an OFDM symbol by a Raised Cosine function, a smoothly decaying waveform is generated as illustrated in FIG. 2B. The end portion of the OFDM symbol #n having the Time Window width and the head portion of the guard interval GI #n+1 of the OFDM symbol #n+1 having the Time Window width are added so that they overlap each other as illustrated in FIG. 2C. By performing the Time Window processing, it is possible to reduce a high-frequency component and interference in an adjacent channel.
However, around the boundary between the OFDM symbols, the waveform of a target OFDM symbol (the OFDM symbol #n) is attenuated and overlaps the waveform of the next OFDM symbol (the OFDM symbol #n+1). As a result, the original waveform of the target OFDM symbol (the OFDM symbol #n) is deteriorated as illustrated in FIG. 2D. This leads to the deterioration of a bit error rate (BER) on the side of a receiver and the degradation of a reception characteristic.
FIG. 3 is a graph illustrating the difference between a transmission spectrum obtained with the Time Window processing and a transmission spectrum obtained with no Time Window processing. The base of a spectrum represented by a curve A obtained with no Time Window processing (a Time Window width=100 [sample]) is high. On the other hand, the base of a spectrum represented by a curve B obtained with the Time Window processing (a Time Window width=100 [sample]) is low since a high-frequency component is reduced.
FIG. 4 is a graph illustrating the difference between a bit error rate (BER) obtained with the Time Window processing and a bit error rate obtained with no Time Window processing. As represented by a curve C obtained with no Time Window processing (a Time Window width=0 [sample]), the larger Ec/N0 (a signal-to-noise ratio) (the lower noise), the smaller BER. On the other hand, as represented by a curve D obtained with the Time Window processing (a Time Window width=100 [sample]), the reduction in BER is suppressed at approximately Ec/N0=13 [dB], and BER is constant and is not changed at values equal to or larger than approximately Ec/N0=19 [dB].
In examples illustrated in FIGS. 3 and 4, the length of an OFDM symbol (before addition of a GI) is 2048 [sample], and the length of a guard interval is 160 or 144 [sample]. Furthermore, one sub-frame is composed of fourteen (No. 0 to No. 13) OFDM symbols. The length of guard intervals of OFDM symbols having Nos. 0 and 7 is 160 [sample], and the length of guard intervals of the other OFDM symbols is 144 [sample]. Sill furthermore, OFDM symbols having Nos. 3 and 10 are reference signals.